Repair of a coating on a turbine component

ABSTRACT

A method is provided for repairing an area of damaged coating on a component of a turbine module in a gas turbine engine, the component formed of a base material having a diffusion coating applied to the base material. The repair may be accomplished in place by directly heating the area to which a touch-up coating material has been applied with a hot gas plasma without the need to place the component in an oven for curing and heat treatment of the touch-up coating applied to the damaged area.

CROSS REFERENCE TO RELATED APPLICATION

This application claims priority to Singapore Patent Application No.201001161-7 entitled “Repair of a Coating on a Turbine Component” filedon Feb. 25, 2010 and is a continuation of U.S. Non-ProvisionalApplication 12/836,107. The content of this application is incorporatedherein by reference in its entirety.

FIELD OF THE INVENTION

This invention relates generally to the repair of turbine components fora gas turbine engine and, more particularly, to a method for repairing adamaged coating on a turbine vane without disassembly of the turbinemodule.

BACKGROUND OF THE INVENTION

Gas turbine engines, such as those used to power modern aircraft or inindustrial applications, include a compressor for pressurizing a supplyof air, a combustor for burning a hydrocarbon fuel in the presence ofthe pressurized air, and a turbine for extracting energy from theresultant combustion gases. Generally, the compressor, combustor andturbine are disposed about a central engine axis with the compressordisposed axially upstream of the combustor and the turbine disposedaxially downstream of the combustor. Air drawn into the engine passesaxially through the compressor into the combustor wherein fuel iscombusted in the air to generate and accelerate combustion gases thatpass through the turbine and out the exhaust nozzle of the gas turbineengine. The combustion gases turn the turbine, which turns a shaft incommon with the compressor to drive the compressor.

As the hot combustion gases pass through the turbine, various turbineelements, such as the turbine stator vanes and turbine rotor blades ofthe turbine, are exposed to the hot combustion gases, which may also becorrosive to the material of which those turbine elements are made. Inorder to protect the turbine elements from oxidation and corrosion dueto exposure to the hot combustion gases, it is conventional practice tocoat various turbine elements with one or more layers of a protectivecoating or coatings. For example, it is known to coat turbine statorvanes in gas turbine engines with an aluminide during the process ofmanufacturing the turbine modules.

The turbine of the gas turbine engine is generally an axially extendingassembly of a plurality of turbine modules mounted to a shaft. Eachturbine module may include one or more turbine stages. Each turbinestage includes a row of stationary airfoils, referred to as the statorvanes, and a row of airfoils, referred to as rotor blades, mounted on arotor disk driven by the airflow passing over the rotor blades. Theturbine may include a high pressure turbine including a plurality ofhigh pressure stages in one or more modules assembled to a common shaftwith a high pressure compressor, as well as a low pressure turbineincluding a plurality of low pressure stages in one or more modulesassembled to a common shaft with a low pressure compressor and/or fan.

In the handling of the turbine modules during shipping, assembling ofthe turbine, and disassembling of the turbine for servicing or overhaul,the protective coating on the turbine elements may be damaged in localareas for example nicked, scrapped, cracked, scored or otherwise removedthereby exposing the base metal of which the turbine element iscomposed. If the damage is deemed sufficient to warrant repair, it iscustomary to repair the damaged coating by removing the module includingthe damaged element and disassembling the module so that the assemblyhaving the damaged element may be replaced or repaired.

To repair a damaged area of coating on a turbine stator vane accordingto conventional practice, it is common to disassemble the turbine moduleto remove the stator assembly containing the damaged vane for service.The coating may then be stripped from the damaged vane, at least in theregion surrounding and including the area of damaged coating, theremainder of the damaged vane exclusive of the stripped area is thenmasked, and a new coating is then applied to the area of damaged coatingand the stripped area. The stator assembly is then placed in a furnaceor oven at a desired temperature for a desired period of time to cureand heat treat the newly applied coating. The turbine module is thenreassembled with the repaired stator assembly. Although effective forrepairing the damaged turbine element, the process is time consuming andlabor intensive as the turbine module must be dissembled to affect thecoating repair since the turbine module itself is too large to be placedin a conventional heat treatment furnace or oven. Further, even if afurnace or oven were large enough to accommodate an entire turbinemodule, the whole surface of the turbine module exclusive of thestripped area to which the new coating has been applied would need to bemasked to reduce the risk of contamination of the undamaged surfaceduring the heat treatment process.

U.S. Pat. No. 6,560,870 discloses a method of applying a diffusion metalcoating to a selective area of a turbine engine component having adeficiency of metal coating. To apply the diffusion metal coating inaccord with the disclosed method, a metal source containing tape ispositioned in contact with the selective area and held in contact withthe selective area using a tape holder that is stable at hightemperatures while the selective area is heated to an effectivetemperature and an effective amount of time to form a metal coating ofpredetermined thickness on the selective area. In the disclosedembodiment, a quartz infrared lamp is used to heat the selective area toa coating temperature of about 1800 F. to about 2000 F. under an inertatmosphere for about 3 to 8 hours.

U.S. Pat. No. 7,115,832 discloses a portable, hand-controlledmicroplasma spray coating apparatus that can be transported to on-sitelocations in the field to apply ceramic and metallic coatings to avariety of workpieces, including gas turbine engine parts. However, theuse of such a microplasma spray coating apparatus to directly spray aplasma coating onto a base material is not generally satisfactory forapplication of coatings that require some degree of diffusion of thecoating material into the base material to be effective.

SUMMARY OF THE INVENTION

A method is provided for repairing an area of damaged coating on acomponent of a turbine module in a gas turbine engine, the componentformed of a base material having a diffusion coating applied to the basematerial, the method comprising the steps of: blending the area ofdamaged coating with an area of coating surrounding the area of damagedcoating; applying a touch-up coating material to the blended area; anddirectly heating the blended area to which the touch-up coating materialhas been applied with a hot gas plasma. The method may include thefurther step of masking an area surrounding the blended area prior tothe step of directly heating the blended area to which the touch-upcoating material has been applied with a hot gas plasma. In anembodiment, the component may comprise a turbine stator vane and thetouch-up coating material may comprise an aluminide coating material.

The step of directly heating the blended area to which the touch-upcoating material has been applied with a hot gas plasma may compriseheating the blended area to which the touch-up coating material has beenapplied with a hot gas plasma stream generated by a hand-controlledplasma gun. The step of directly heating the blended area to which thetouch-up coating material has been applied with a hot gas plasma maycomprise heating the blended area to which the touch-up coating materialhas been applied with a hot gas plasma stream generated by ahand-controlled microplasma coating spray gun having a gas nozzle and apowder coating injector, the powder coating injector being deactivatedduring the heating step.

In an aspect, a turbine vane formed of a base material having adiffusion coating of a coating material thereon includes an area ofrepaired coating. The area of repaired coating comprises a touch-upcoating material applied to an area of damaged coating and partiallydiffused into the base material by directly heat treating the appliedtouch-up coating material with a hot gas plasma. In an embodiment, theapplied touch-up coating material has been heat treated with a hot gasplasma stream generated by a hand-controlled plasma gun. In anembodiment, the applied touch-up coating material has been heat treatedwith a hot gas plasma stream generated by a hand-controlled microplasmacoating spray gun having a gas nozzle and a powder coating injector, thepowder coating injector being deactivating during the heat treatment.The touch-up coating material may comprise an aluminide coatingmaterial.

BRIEF DESCRIPTION OF THE DRAWINGS

For a further understanding of the disclosure, reference will be made tothe following detailed description which is to be read in connectionwith the accompanying drawing, wherein:

FIG. 1 is a schematic view of a longitudinal section of an exemplaryembodiment of a turbofan gas turbine engine;

FIG. 2 is a perspective view of an exemplary turbine vane having an areaof damaged coating;

FIG. 3 is a perspective view of the turbine vane of FIG. 2 having ablended area surrounding the area of damaged coating;

FIG. 4 is a perspective view of the turbine vane of FIG. 3 having atouch-up coating applied to the blended area and the area of damagedcoating;

FIG. 5 is a partial schematic, partial perspective view illustrating theheat treatment of a touch-up coating in accordance with the methoddisclosed herein; and

FIG. 6 is a flow chart illustrating an exemplary embodiment of a methodof repairing a damage coating on a turbine element.

DETAILED DESCRIPTION OF THE INVENTION

There is depicted in FIG. 1 an exemplary embodiment of a turbofan gasturbine engine, designated generally as 100, that includes, fromfore-to-aft longitudinally about a central engine axis 150, a fan 102, alow pressure compressor 104, a high pressure compressor 106, a combustormodule 120, a high pressure turbine 108, and a low pressure turbine 110.A nacelle forms a housing or wrap that surrounds the gas turbine engine100 to provide an aerodynamic housing about gas turbine engine. In theturbofan gas turbine engine 100 depicted in the drawings, the nacelleincludes, from fore to aft, the engine inlet 132, the fan cowl 134, theengine core cowl 136 and the primary exhaust nozzle 140. It is to beunderstood that the method described herein is not limited inapplication to the depicted embodiment of a gas turbine engine, but isapplicable to other types of gas turbine engines, including other typesof aircraft gas turbine engines, as well as industrial and powergeneration gas turbine engines.

The method for repairing an area of damaged coating on a component of aturbine module in a gas turbine engine disclosed herein will bedescribed in reference to the repair of an area of damaged coating on aturbine stator vane. Referring now to FIGS. 2-4, there is depicted asegment of a turbine stator assembly 20 of the low pressure turbine 110.The turbine stator assembly 20 includes a plurality of stator vanes 22disposed at circumferentially spaced intervals about and extendingradially between a radially inboard rim 24 and a radially outboard rim26 circumscribing the radially inboard rim 24. Each of the stator vanes22 is formed of a base metal coated with a layer of protective coating,such as an aluminide coating. For purposes of illustration, the depictedsegment includes three stator vanes 22, one of which has an area 30 ofdamaged coating.

To repair the area 30 of damaged coating in accord with the methoddescribed herein, the area 30 of damaged coating is blended with an area40 of undamaged coating surrounding the area of damaged coating. Theblending may be accomplished by grit blasting the area 30 and area 40followed by abrading the areas 30 and 40 with an abrasive. The abrasionblending may be performed, for example, by hand or by power with ahand-controlled power tool with abrasive-impregnated wheels, stones,and/or pads. Other forms of blending may be used that are also effectiveto remove any previously diffused portion of the coating remainingwithin the areas 30 and 40.

After completion of the blending process, a touch-up coating 50 isapplied to the blended areas 30 and 40. The touch-up coating may beapplied as a slurry brushed onto the blended areas 30 and 40.Alternatively, the touch-up coating 50 may be applied in a tape form.The tape may be impregnated with the touch-up material and may be cut tothe shape of the blended areas 30 and 40. The touch-up coating may, forexample, comprise an aluminide material.

The touch-up coating 50 applied to the blended areas 30 and 40 must beheated to first cure the touch-up coating 50 to partially diffuse thetouch-up coating into the base metal of the stator vane 22. To properlycure the applied touch-up coating 50, the blended areas 30 and 40 mustbe heated to a temperature in the range of 155 C. to 163 C. (275 F. to325 F.) for a period not less than fifteen minutes. Following curing,the blended areas 30 and 40 of the stator vane 22 must be heat treatedat a higher temperature and for a longer period of time. For example,the heat treatment may be carried at a temperature in the range of 871C. (1600 F.) for a period of at least four hours. The area surroundingthe blended areas 30, 40 to which the touch-up coating has been appliedmay be masked with a protective tape 70 prior to heating the appliedtouch-up coating 50.

Instead of placing the stator assembly 20 in an oven for curing of theapplied coating 50 and subsequent heat treatment, in the methoddisclosed herein, the necessary heating is accomplished in place bydirectly heating the blended areas 30 and 40 with a gas plasma jet 60,as illustrated in FIG. 5. The inert gas plasma jet may be generatedusing a portable microplasma spray coating apparatus having the coatingpowder dispensing function disabled. As illustrated schematically inFIG. 5, the microplasma spray coating apparatus 80, represented by thedashed outline, includes an arc gas emitter 82, an anode 84, a cathode86, and a coating powdered material injector 88. In operation of themicroplasma spray coating apparatus 80 in a conventional manner, anelectric arc is generated between the anode 84 and the cathode 86. Theplasma stream 60 is generated as the arc gas, such as, but not limitedto, argon, injected from the emitter 82 passes through the electric arcgenerated between the anode 84 and the cathode 86. For a more detaileddiscussion of a portable microplasma spray coating apparatus, referenceis made to U.S. Patent Application Publication No. US 2006/0093748,published May 4, 2006 the disclosure of which is hereby incorporatedherein by reference in its entirety.

When the microplasma spray coating apparatus 80 is used in aconventional manner to apply a plasma spray coating to a workpiece, apowdered coating material is dispensed through the injector 88 into theplasma stream 60 and transported in the plasma stream 60 to theworkpiece. However, in accord with an aspect of the method disclosedherein, the portable microplasma spray coating apparatus 80 may be usedto generate the hot gas plasma stream 60 and direct the hot gas plasmastream 60 directly impinge upon the touch-up coating 50 applied to theblended areas 30 and 40 by disabling the powdered coating materialdispensing function so that no powdered coating material is dispensedinto the hot gas plasma stream. With the powdered coating materialdispensing function disabled, the portable microplasma spray coatingapparatus 80 generates only a pure hot gas plasma stream 60, for examplean argon shrouded hot gas flame. Although the hot gas plasma stream maytemporary reach temperatures in excess of 20,000 F. as the arc gas isionized in passing through the electric arc, the hot gas plasma stream60 rapidly cools after passing through the electric arc.

The temperature of the hot gas plasma stream 60 directly impinging uponthe touch-up coating material applied to the blended areas 30 and 40 onthe turbine stator vane 22, as depicted in FIG. 5, may be controlled byadjusting the gas feed rates and the distance traveled by the hot gasplasma stream 60. Typically, the gas feed rate will range from about 3.8to about 38 liters/minute (about 1 to about 10 gallons/minute). Theportable microplasma spray coating apparatus 80 may be hand-controlledso as to selectively position the gas emitter 82 at a desired distancefrom the blended areas 30 and 40 on the turbine stator vane 22,typically at a distance ranging from about 76 to about 200 millimeters(about 3 to about 8 inches). The portable microplasma spray coatingapparatus 100 may be hand-controlled so as to selectively control thetime for which the hot gas plasma stream 60 actually heats the touch-upcoating 50 on the blended areas 30 and 40.

A suitable portable microplasma spray coating apparatus for use indirectly heating the blended area 30, 40 to which the touch-up coatingmaterial 50 has been applied with hot gas plasma in accord with anaspect the method disclosed herein is the Compact Plasma Spray (CPS)System, a portable thermal spray system commercially available fromSulzer Metco (US) Inc. of Westbury, N.Y., USA. The Sulzer Metco CPSsystem uses argon as the plasma gas and has a maximum power of 3 kW, amaximum DC amperage of 50 amps, and a maximum DC voltage of 40 volts.

Referring now to FIG. 6, a process flow chart is depicted generallyillustrating an exemplary embodiment of the method disclosed herein.Initially, an area of damaged coating 30 and an area 40 surrounding thearea of damaged coating are blended. After completion of the blendingstep, a touch-up coating material 50 is applied to the blended area 30,40, such as by application of a slurry of coating material orapplication of a tape impregnated with coating material. The areasurrounding the area 30, 40 to the touch-up coating material 50 has beenapplied is masked. The microplasma spray coating apparatus 80 is thenenergized and the powdered coating dispensing function is disabled. Itis to be understood that the powdered coating dispensing function couldbe disabled either before or after the microplasma spray coatingapparatus 80 is energized. The microplasma spray coating apparatus isselectively positioned a desired distance from the workpiece and aimedat the area of touch-up coating 50. The flow of gas through the emitter82 is then initiated at the desired gas flow rate to affect directheating of the blended area 30, 40 to which the touch-up coatingmaterial 50 has been applied with a hot gas plasma to partially diffusethe applied touch-up coating material into the base material of theworkpiece. The use of the pure hot gas plasma stream generated by themicroplasma coating apparatus 80 with its powdered dispensing functiondisabled substantially reduces the risk of coating contamination.

The method disclosed herein allows for the in-situ repair of localizedareas of damaged coating on a turbine component of a gas turbine enginewithout dissembling the turbine module. By eliminating the need todisassemble one or several stages of a turbine to remove a turbinemodule having a component with a localized area of damaged coating,application of the method disclosed herein will reduce maintenance costand reduce repair turnaround time. The application of the hot gas plasmastream directly to the localized area to which a touch-up material hasbeen applied provides sufficient heat energy to partially diffuse thecoating material into the base material so that coating orientation hasno significant effect on tensile or fatigue properties. Additionally,the likelihood of bond defects or abnormal grain structures orporosities is minimized. The method disclosed herein may be used torepair localized areas of damaged coating on components having simple orcomplex structures.

The terminology used herein is for the purpose of description, notlimitation. Specific structural and functional details disclosed hereinare not to be interpreted as limiting, but merely as basis for teachingone skilled in the art to employ the present invention. While thepresent invention has been particularly shown and described withreference to the exemplary embodiments as illustrated in the drawing, itwill be recognized by those skilled in the art that variousmodifications may be made without departing from the spirit and scope ofthe invention. Those skilled in the art will also recognize theequivalents that may be substituted for elements described withreference to the exemplary embodiments disclosed herein withoutdeparting from the scope of the present invention.

Therefore, it is intended that the present disclosure not be limited tothe particular embodiment(s) disclosed as, but that the disclosure willinclude all embodiments falling within the scope of the appended claims.

1. A turbine vane for a gas turbine engine, the turbine vane formed of abase material having a diffusion coating of an aluminide coatingmaterial thereon including an area of damaged aluminide coating repairedby: blending the area of damaged aluminide coating with an area ofundamaged aluminide coating surrounding the area of damaged aluminidecoating by grit blasting; applying an aluminide touch-up coatingmaterial to the blended area; and curing the aluminide touch-up coatingapplied to the blended area by heating the aluminide touch-up coating toa first temperature with a hot gas plasma stream generated by a portablemicroplasma spray coating apparatus.
 2. A turbine vane as recited inclaim 1 further comprising directly heating the blended area to whichthe aluminide touch-up coating has been applied to a second temperaturewith the hot gas plasma stream generated by the portable microplasmaspray coating apparatus, the second temperature being higher than thefirst temperature.
 3. A turbine vane as recited in claim 2 wherein thestep of heating the blended area to which the aluminide touch-up coatingmaterial has been applied with the hot gas plasma stream generated bythe portable microplasma spray coating apparatus comprises the step ofheating the blended area to which the aluminide touch-up coatingmaterial has been applied with the hot gas plasma stream generated bythe portable microplasma spray coating apparatus having a gas nozzle anda powder coating injector, the powder coating injector being deactivatedduring the heating step.
 4. A turbine vane as recited in claim 1 furthercomprising the step of abrading said grit-blasted area with an abrasivedevice.
 5. A turbine vane as recited in claim 1 wherein the step ofapplying an aluminide touch-up coating material to the blended areacomprises applying a slurry including the aluminide touch-up coatingmaterial to the blended area.
 6. A turbine vane as recited in claim 1wherein the step of applying an aluminide touch-up coating material tothe blended area comprises applying a tape carrying the aluminidetouch-up coating material to the blended area.
 7. A turbine vane asrecited in claim 1 further comprising controlling a temperature of thehot gas plasma stream by adjusting a gas feed rate of the portablemicroplasma spray coating apparatus.